My son has been trying to put on some weight... He's 17, 5' 6.5", and was 105 four weeks ago. Yesterday he was up to 111.
Smoothies have played a big role in that. He adds a bit of coconut milk (the kind from a can), nut butter, and protein powder (Amazing Grass Original) along with the fruit, bananas and greens.
It's much easier to drink your calories than it is to eat them.
He's also been doing a lot of calesthenics - pullups and push-ups, mostly. Nothing that gets his heart rate up too much.
Vegan cheese is a good thing to add too - lots are made with oil, and I've been making him A LOT of pasta with vegan meatballs.
I will even pick up some french fries for him at Jack in the box sometimes.
I don't like this way of saying for long term, but he needed the weight now, and calorie dense foods are what's going to do it. There good thing is, when you're older, your more likely to have the opposite problem.
Where are you located? If you're in the US, I could point you to some high calorie vegan foods at Costco...
Glad to see your still here. Some people just have a super high metabolism. Keep up the smoothies and try some muscle building exercises. You got this!